In connection with such twin-screw kneaders it is known to provide double-flight kneading disks, whose crests move in an approximately clearance-free manner past the casing wall. The disadvantage of these known twin-screw kneaders is that they do not have an optimum melting capacity for certain applications. The kneading zone formed from positively displaced kneading disks, i.e. comprising kneading disks arranged in a conveying-effective manner do not bring about a complete melting of the standard thermoplastic materials, such as polyolefins, styrene polymers, polyesters, polyethylenes, etc. In order to achieve this, it is necessary to provide baffle members in the form of constrictors or return conveying screws or kneading members. This has a negative influence on the feed performance, because material feed is prevented by the back pressure resulting from the baffle members. This is particularly the case when pulverulent materials with a low bulk density are used, so that the degree of filling is impaired. It also leads to unfavourable mechanical effects and specifically to local high wear, because the melting mainly takes place directly upstream of the baffle member, so that locally high torques are obtained in the melting zone.
It is also known to so correct such double-flight kneading disks on their crests and flanks that a clearance is formed between the kneading disks and the casing wall. This reduces the aforementioned disadvantages, but they are fundamentally still present. An additional disadvantage is the lack of self-cleaning.
It is known from European Patent Specification No. 0037984 in connection with a co-rotating twin-screw kneader to provide kneading disks constructed as eccentric disks with a circular cross-section. In the penetration area of the bores, a clearance is provided between two adjacent kneading disks, whilst there is also a clearance with respect to the casing wall and which is larger than the first-mentioned clearance. As these eccentric disks have a circular cross-section, they must be spaced from the casing wall in order to be able to operate.
European Patent Specification No. 0002131 discloses a twin-screw kneader with double-flight screws, in which in each case one screw thread runs in an approximately clearance-free manner with respect to the casing wall, whilst the other screw thread has a definite clearance with respect to the said wall. The two screws are associated with one another in such a way that a self-cleaning effect occurs. Each of the screws can have two to four flights.
A twin-screw kneader with co-rotating screw shafts is also known from U.S. Pat. No. 4,131,371, in which the screw threads are constructed in such a way that the profile centres are arranged on a helix, which winds around the screw axis. Such a construction is highly complicated from the design standpoint.